Control of Tundra Plant Allocation Patterns and Growth 173 



In spite of a 15-fold variation in soil solution phosphate concentra- 

 tion and a 6-fold variation in leaf phosphorus between microtopographic 

 units, leaf phosphorus concentration shows no correlation with photo- 

 synthetic rate (Chapter 4), suggesting that the photosynthetic apparatus 

 of a graminoid is relatively insensitive to changes in phosphate concen- 

 tration. In contrast, shoot production is positively correlated with 1) 

 availabihty of soil phosphorus, 2) capacity of the plant to absorb avail- 

 able phosphorus (Chapin 1978), and consequently 3) concentration of 

 phosphorus in leaves (Table 5-3). The correlations suggest an intimate as- 

 sociation between the phosphorus nutrition of the plant and growth un- 

 der natural conditions in the field. However, the nature of the relation- 

 ship between phosphorus nutrition and growth requires further study. 



The maximum nitrogen and phosphorus contents of graminoids at 

 Barrow are as high as or higher than those of native temperate zone 

 graminoids (Chapin et al. 1975). This might be anticipated because of a 

 low percentage of structural material. Nitrogen requirements may be 

 high in tundra plants because of high enzyme complements, as discussed 

 previously. A high phosphorus complement may result from 1) a high in- 

 cidence of polyploidy (Johnson and Packer 1965) and consequently high 

 DNA content, and 2) high concentrations of membrane phosphoHpid to 

 support metabolism and convey cold tolerance (de la Roche et al. 1972, 

 Thomson and Zalik 1973, Kedrowski and Chapin 1978). Dupontia prob- 

 ably has a higher proportion of its phosphorus tied up in structural mate- 

 rial, DNA and phospholipid than do temperate plants (Figure 5-15). The 

 total phosphorus complement of leaves varies considerably through the 

 season and between habitats of different phosphorus status. Plants with 

 low phosphorus content may have as much as 85 Vo of their phosphorus 

 complement structurally bound and have essentially the same phos- 

 phorus composition as standing dead material (Figure 5-15). 



Along a nutrient gradient nitrogen concentration of mature leaves is 

 not correlated with photosynthetic potential. The percentage change in 

 carboxylation activity from early season to mid-season and then from 

 mid-season to end-of-season is greater than the corresponding change in 

 nitrogen content, indicating that early and late in the season a higher pro- 

 portion of shoot nitrogen is bound as structural protein and as nonpho- 

 tosynthetic enzymes than at mid-season (Figure 5-16). This may partially 

 explain the low photosynthetic rates early in the growing season (Chapter 

 4). The parallel decrease in carboxylation activity and total nitrogen con- 

 tent in the latter half of the growing season suggests that ribulose 

 diphosphate carboxylase is broken down more rapidly than it is synthe- 

 sized after mid- to late July and that the nitrogenous breakdown pro- 

 ducts are translocated out of the shoot at this time. The decrease in total 

 carboxylation activity and shoot nitrogen after mid- to late July indicates 

 a strong selection for early senescence and downward translocation of 



